Device for determining density and concentration of visible constituents in fluids

ABSTRACT

An arrangement is proposed for determining the density and concentration of visible constituents in fluids, particularly for measuring the turbidity of motor vehicle exhaust gases. A measuring chamber (10) which can receive the fluid to be measured is provided at two oppositely disposed points a first light detector (16) and a light source (15) which directs a light beam at the light detector (16). This arrangement allows the opacimetric measuring method in which the light attenuation is evaluated as a function of the turbidity. In addition, a second light detector (19) is arranged outside of and lateral to the radiation path through the measuring chamber (10) for capturing scattered light. This allows use of the scattered light method in which the scattered light portion is captured as a measure for the concentration of the light-scattering particles in the investigated medium. In an evaluation arrangement, the measuring signals (turbidity and scattered light) can be evaluated simultaneously or selectively.

BACKGROUND OF THE INVENTION

The invention relates to an arrangement for determining the density andconcentration of visible constituents in fluids, particularly formeasuring the turbidity of motor vehicle exhaust gases. Moreparticularly, the invention relates to a density determining arrangementof the type having a measuring chamber through which the exhaust gasflows, which measuring chamber is provided at two oppositely disposedpoints with a first light detector and a light source, and having anevaluation arrangement which evaluates the signal generated by the lightdetector as a measure of density and concentration.

Such a turbidity measuring device or opacimeter measures the attenuationof a light beam through a measuring chamber which is filled with thefluid to be measured or through which the fluid flows, with the lightattenuation taken as the measure for the turbidity. The evaluation takesplace according to the Lambert-Beer law which describes the lightattenuation during passage through absorbing media. The assignee of thisapplication distributes such a turbidity measuring device under thedesignation RTT 100/100 which serves to measure flue gas or soot inmotor vehicle exhaust gases. The drawback of the known arrangements isthat this turbidity measuring method fails for exhaust gases with lowsoot concentrations, as they are encountered with ever increasingfrequency in engines or combustion systems of a more recent design. Bymeans of structural or combustion engineering measures, the sootemission is reduced to such an extent that the accuracy of theopacimetric measuring methods is no longer sufficient. The reason forthe limited accuracy for low soot concentrations lies in the principleof the measuring method, which is based on measuring a differencebetween a brightness signal and an attenuated signal, i.e., a differencebetween two signals which are almost identical in size.

From GB 22 52 621, a scattered light measuring method is known, whereinfirst a light beam is also guided into a fluid to be measured. The lightportion which is scattered in different directions is captured andevaluated. In scattered light methods, a measuring signal is obtainedwhich is proportional to the concentration of the absorbing medium. Thisscattered light method is suited particularly for low sootconcentrations, but it is inferior to the opacimetric measuring methodin the range of higher concentrations.

SUMMARY OF THE INVENTION Advantages of the Invention

The arrangement according to the invention is characterized in that asecond light detector is arranged outside lateral to the radiation paththrough the measuring chamber for capturing scattered light, and in thatthe evaluation arrangement evaluates the signal generated by the secondlight detector independently of the signal generated by the first lightdetector also as a measure for the density and concentration. Such anarrangement has the advantage that the advantages of the two knownmeasuring methods are combined with one another, with a single measuringchamber being required and with it being possible to carry out themeasurements of both methods simultaneously. In this manner,considerably greater accuracy is obtained over the entire range with ameasuring complexity that is hardly any more extensive, and very highand very low concentrations can be measured with great accuracy.

Advisably, the measuring chamber is configured as a measuring tubethrough which the fluid to be measured flows during measuring, with thefirst light detector and the light source preferably being arranged atthe two end faces of the measuring tube so that a measuring section isobtained which is as long as possible.

The second light detector (scattered light) is arranged in anadvantageous manner substantially in the center between the light sourceand the first light detector on the side of the measuring chamber sothat the scattered light can be captured at a small distance.

In order to be able to selectively supply the signals of the two lightdetectors to a display arrangement, the evaluation arrangement ispreferably provided with a change-over device. This change-over devicemay be configured for manual change-over or be controlled automatically,with a control device being particularly suited in the latter case whichsupplies the measuring signals of the second light detector to thedisplay arrangement below a predeterminable measuring signal level ofthe first light detector by changing over the change-over device. Thisensures that, at low soot concentrations, the measurement takes place byway of the scattered light method which works with greater accuracy inthis range.

Of course, it is also possible to use both measuring methodssimultaneously, in which case the evaluation arrangement has two displayarrangements for the signals of the two light detectors. This offers thepossibility of a constant comparison between the two measuring methods.

For the implementation of correlation measurements, it is advisable toprovide a comparator device for the signals of the two light detectorsso that the comparison can be carried out automatically. By way of thecomparator unit, a normalizing device for normalizing the signal of theone light detector as a function of the signal of the other lightdetector can be provided in an advantageous manner so that comparablemeasuring results are obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The single FIGURE shows a schematic representation of a measuring tubeand a block diagram of an evaluation arrangement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS.

A measuring tube 10 employed as a measuring chamber has a laterallydisposed inlet 12 at one end region, which inlet is provided with acalibration valve 11, and a laterally disposed outlet 13 at the endregion which lies opposite, which outlet is connected to a pump 14.

A light source 15 is arranged at the inlet-side end face of themeasuring tube 10, and a first light detector 16, which normally is aphotodetector, is arranged at the oppositely-disposed end face. Anoptical arrangement 17, shown schematically as a lens, concentrates thelight beam, which is sent through the measuring tube 10 by the lightsource 15, on the first light detector 16. A light source-end opticalarrangement 18, shown schematically as a reflector, serves to orient thelight beam through the measuring tube 10. In the interest ofsimplification, optical elements which may be additionally required atthe end faces of the measuring tube 10, if applicable, are not shown.The optical measuring section is limited in a known manner by measuringwindows 8, 9. If the light source 15 is configured as a laser diode,optical arrangement can be eliminated to a large extent or altogether.

A second light detector 19 is arranged laterally on the outside of themeasuring tube 10 between the light source 15 and the first lightdetector 16, with the second light detector capturing scattered lightfrom a limited space region, with an optical arrangement 20, which isagain shown schematically as a lens, supplying the scattered lightgenerated in the measuring tube 10 by the light beam to the second lightdetector 19.

The measuring signals generated in the two light detectors 16, 19 aresupplied to signal processing stages 21, 22, respectively, which may beconfigured, for example, as amplifiers or level matching stages.Furthermore, such signal processing stages may also comprise filters forlocking out interfering signals or background noise. The output of thesignal processing stage 21 can be connected via a change-over switch 23with a measurement output unit such as a display or data acquisitionarrangement 24 which, in the simplest case, may be configured as adigital or analog measuring instrument or as a screen displayarrangement. The output of the other signal processing stage 22 is alsoconnected with the change-over switch 23 via a normalizing unit 25 sothat the processed signals of the two light detectors 16, 19 can besupplied selectively to the measurement output unit 24 by means of thischange-over switch 23.

Furthermore, the output signals of the signal processing stages 21, 22are supplied to a comparator unit 26 whose output signal acts upon thenormalizing unit 25. In addition, the output signal of the signalprocessing stage 21 is supplied to a level control unit 27 by means ofwhich the change-over switch 23 is placed into the shown switchingposition below a predeterminable signal level of the output signal ofthe signal processing stage 21, in which position the measurement outputunit 24 receives the signals of the second light detector 19.

For the measurement, the fluid to be measured, for example, the exhaustgas of a motor vehicle, is guided through the measuring tube 10 in thatthe pump 14 works while the calibrating valve 11 is open. To this end,the exhaust pipe of a motor vehicle, not shown, for example, isconnected with the calibrating valve 11 by a hose. For the zero balance,the calibrating valve 11 is changed over to the flushing position sothat a flushing medium can be guided through the measuring chamberinstead of the exhaust gas. Precipitates on the measuring windows canthus be detected and considered in the measuring result.

The light beam traveling through the measuring tube 10 from the lightsource 15 to the first light detector 16 is attenuated to a greater orlesser degree as a function of the soot constituents in the exhaust gasor of the turbidity of the fluid to be measured. The light attenuationduring passage through absorbing media is evaluated according to theLambert-Beer law. Via the change-over switch 23, the measuring signalevaluated in the signal processing stage 21 reaches the measurementoutput unit 24 where it indicates the degree of turbidity or the sootcontent in the exhaust gas. If the turbidity drops below apredeterminable level, the level control unit 27 responds and switchesthe change-over switch 23 into the switch position which is shown. Inthis manner, measuring signals of the second light detector 19 now reachthe measurement output unit 24. The scattered light is captured by thesecond light detector 19, which scattered light is proportional to theconcentration of the absorbing medium, i.e., for example, totheconcentration of the soot in the exhaust gas. This measuring methodis more accurate below a predeterminable turbidity than the opacimetricmeasuring method by way of the first light detector 16.

In order to obtain comparable measuring results, namely to make thetransition between the different measuring methods a continuous one, theoutput signals of the two signal processing stages 21, 22 are suppliedto a comparator unit 26 and control the normalizing unit 25 as afunction of this comparison or of the respective deviation; theprocessed signals of the second light detector 19 are matched to thoseof the first light detector 16 by means of the normalizing unit. Thismay take place, for example, in that the respective measured curves arecompared and matched with one another. In this manner, correlationmeasurements between the two methods can be carried out. Once limitvalues have been determined and set for the one system, these may thusbe transferred to the other measuring method. By combining thecomparator unit 26 with the normalizing unit 25, the respective measuredvalues or measured curves can also be corrected. In individual cases, anormalizing unit may also be connected downstream of the signalprocessing stage 21 for this purpose.

By switching the change-over switch 23 back and forth, the measuredvalues of the two methods can be displayed simultaneously on themeasurement output unit 24. Of course, this may also take place in thatboth measuring lines are supplied to the measurement output unit 24 orto two separate display/data acquisition units without using achange-over switch 23.

The evaluation arrangement described may preferably be configured as amicroprocessor.

I claim:
 1. An arrangement for determining density and concentration ofvisible constituents in motor vehicle exhaust gas, comprising:a lightsource (15); a first light detector (16); a measuring chamber (10)through which flows the exhaust gas, the light source (15) and the firstlight detector (16) being provided at two oppositely disposed points ofthe measuring chamber (10); an evaluation means for evaluating a signalgenerated by the first light detector (16) as a first measure of thedensity and concentration; and a second light detector (19), disposedoutside a radiation path through the measuring chamber (10) between thelight source (15) and the first light detector (16), for capturingscattered light, wherein the evaluation means further comprises meansfor evaluating a signal generated by the second light detector (19)independently of the signal generated by the first light detector (16)as a second measure of the density and concentration, and normalizingmeans (25) for normalizing the signal of one of the first and secondlight detectors (16, 19) as a function of the signal of the other of thefirst and second light detectors (16, 19) so as to match the first andsecond measures of the density and concentration.
 2. An arrangementaccording to claim 1, wherein the measuring chamber (10) is configuredas a measuring tube through which the motor vehicle exhaust gas flowswhile the density and concentration of visible constituents therein aredetermined.
 3. An arrangement according to claim 2, wherein themeasuring tube (10) has two end faces, and wherein the first lightdetector (16) and the light source (15) are arranged at the two endfaces of the measuring tube (10).
 4. An arrangement according to claim1, wherein the measuring chamber (10) has a side, and wherein the secondlight detector (19) is arranged substantially in the center between thelight source (15) and the first light detector (16) on the side of themeasuring chamber (10).
 5. An arrangement claim 1, wherein theevaluation means further comprises a change-over unit (23) whichselectively supplies the signals of the first and second light detector(16, 19) to a display or data acquisition arrangement.
 6. An arrangementaccording to claim 5, wherein the evaluation means further comprisescontrol unit means (27) for controlling the change-over device (23) sothat the change-over device (23) supplies a measuring signal of thesecond light detector (19) to the display or data acquisitionarrangement below a predeterminable measuring signal level of the firstlight detector (16) or of the second light detector (19).
 7. Anarrangement according to claim 5, wherein the change-over device (23) isconfigured such that it can be changed over manually.
 8. An arrangementaccording claim 1, further comprising a measurement output unit (24)connected to the evaluation means, the measurement output unit (24)comprising display or data acquisition arrangements, or one display ordata acquisition arrangement having two display options for the signalsof the first and second light detectors (16, 19).
 9. An arrangementaccording claim 1, wherein the evaluation means further comprisescomparator means (26) for implementing correlation measurements for thesignals of the first and second light detectors (16, 19).
 10. Anarrangement according to claim 9, wherein the normalizing means (25) iscontrolled by the comparator means (26).
 11. An arrangement formeasuring turbidity of a fluid, comprising;a first light detector; meansfor shining a beam of light on the first light detector, the beam oflight passing through the fluid; a second light detector which isdisposed out of the beam of light and which receives light that has beenscattered by the fluid; a measurement output unit; and means forconveying a signal derived from the first light detector to themeasurement output unit if the turbidity of the fluid is above apredetermined level and for conveying a signal derived from the secondlight detector to the measurement output unit if the turbidity of thefluid is below the predetermined level.
 12. An arrangement according toclaim 11, further comprising a measuring chamber having an inlet for thefluid and an outlet for the fluid, the beam of light passing through themeasurement chamber.
 13. An arrangement according to claim 12, whereinthe fluid is exhaust gas from a motor vehicle, and further comprising acalibration valve for selectively conveying the exhaust gas or acalibration gas to the inlet of the measurement chamber.
 14. Anarrangement according to claim 11, wherein the means for conveying asignal comprises an electrically controlled switch, the electricallycontrolled switch being controlled by a control signal derived from oneof the first and second light detectors.
 15. An arrangement according toclaim 11, wherein the means for conveying a signal comprises normalizingmeans for adjusting one of the signal derived from the first lightdetector and the signal derived from the second light detector so thatthey have a common signal level at the predetermined level of turbidity.